The expansion of liquid CO.sub.2 in a so-called snow horn for the purpose of producing solid CO.sub.2 particles is a well-known practice, examples of such systems being generally shown in U.S. Pat. Nos. 3,492,829 and 3,786,644. Such particles or snow flakes are generally used as a consumable refrigerant, being allowed to fall by gravity upon a product (such as meat) where they dissipate and exchange thermal energy with the product so as to cool it.
Generally, liquid CO.sub.2 is stored at an equilibrium temperature of about 0.degree. F. and pressure of about 300 psig. When expanded to atmospheric pressure, no more than a theoretical maximum of 47 percent by weight of solid dry ice may be obtained, the remainder of the material going indirectly from the liquid state to a gas having a specific gravity of 1.53 and a temperature of minus 109.degree. F. Since this waste gas is much heavier than air, it will accumulate at low points in the work area and will, particularly in the presence of water vapor, create a thick white fog. Although CO.sub.2 gas is not toxic per se, it can cause hyperventilation and similar physiological effects in workers exposed to it for prolonged periods of time. Therefore, governmental regulations require that it be held to a certain minimal level in the working area for employee safety, thereby requiring that appropriate ventilation steps by taken.
The dry ice crystals formed after liquid CO.sub.2 has been expanded through an orifice are of little value since the bulk of them will become entrained in the waste gas and be carried away as it is exhausted from the work area. Therefore, it is necessary to agglomerate such individual crystals into flakes that will fall by gravity out of the waste gas as dry ice snow. This is usually accomplished by impinging the crystals upon a surface located close to the expansion orifice, such as the inside surface of a snow horn, thereby allowing the flakes to form and fall out of the open bottom of the device.
Although it is known that the size and number of orifices and the shape of the snow horn affect the production rate and velocity of the dry ice snow leaving the snow horn, conventional snow horns have not been able to approach the theoretical maximum solid particle production ratio possible. When converting liquid CO.sub.2, such conventional snow horns typically achieve a conversion ratio of less than 40 percent solid dry ice by weight.